The Soviet Union, officially known as the Union of Soviet Socialist Republics (USSR), was a federal sovereign state in northern Eurasia that existed from 1922 to 1991. Nominally a union of multiple national Soviet republics, in practice its government and economy were highly centralized. The country was a one-party state, governed by the Communist Party with Moscow as its capital in its largest republic, the Russian Soviet Federative Socialist Republic (Russian SFSR). Other major urban centers were Leningrad, Kiev, Minsk, Tashkent, Alma-Ata, and Novosibirsk. It spanned over 10,000 kilometers (6,200 mi) east to west across 11 time zones, and over 7,200 kilometers (4,500 mi) north to south. Its territory included much of Eastern Europe, as well as part of Northern Europe and all of Northern and Central Asia. It had five climate zones: tundra, taiga, steppes, desert and mountains.
The Soviet Union had its roots in the 1917 October Revolution, when the Bolsheviks, led by Vladimir Lenin, overthrew the Russian Provisional Government which had replaced the autocratic regime of Tsar Nicholas II during World War I. In 1922, after a civil war ending in the Bolsheviks' victory, the USSR was formed by a treaty which united the Russian, Transcaucasian, Ukrainian and Byelorussian republics. Following Lenin's death in 1924 and a brief power struggle, Joseph Stalin came to power in the mid-1920s. Stalin formalized the Communist Party's ideology of Marxism–Leninism and replaced the market economy with a command economy which led to a period of rapid industrialization and collectivization. During this period, rapid economic development resulted in dramatic improvements in the average standard of living, particularly in urban areas. Despite these improvements, significant tragedies also occurred. In addition to drought, which was a primary factor in a long history of regularly occurring famines in the region, agricultural collectivization contributed to a major famine in 1932-33, causing millions of deaths. Political paranoia fermented, especially after the rise of the Nazis in Germany in 1933, culminating in the Great Purge, during which hundreds of thousands of persons accused of spying or sabotage were arrested and executed without trial.
On 23 August 1939, after unsuccessful efforts to form an anti-fascist alliance with Western powers, the Soviets signed the non-aggression agreement with Nazi Germany. After the start of World War II, the formally neutral Soviets invaded and annexed territories of several Eastern European states, including Poland and the Baltic states. In June 1941, Germany invaded the Soviet Union, opening the most extensive and bloodiest theater of war in history. Soviet casualties accounted for the highest proportion of the war in the effort of acquiring the upper hand over the Axis forces at intense battles such as Stalingrad and Kursk. In most of the territories occupied by the Red Army after its westward advance, local communists assumed power and formed governments allied with the Soviets. The post-war division of Europe into capitalist and communist halves led to increased tensions with the United States-led Western Bloc, known as the Cold War. Stalin died in 1953 and was eventually succeeded by Nikita Khrushchev, who in 1956 denounced Stalin and began a period of liberal reforms known as de-Stalinization. The Cuban Missile Crisis occurred during Khrushchev's rule, which was among the many factors that led to his removal in 1964. In the early 1970s, there was a brief détente of relations with the United States, but tensions resumed with the Soviet–Afghan War in 1979. In 1985, the last Soviet premier, Mikhail Gorbachev, sought to reform and liberalize the economy through his policies of glasnost (openness) and perestroika (restructuring). These policies caused political instability arising from nationalist and separatist movements. In 1989, Soviet-allied states in Eastern Europe were overthrown in a wave of revolutions which ended communist rule.
As part of an attempt to prevent the country's collapse, a referendum was held in March 1991, boycotted by three republics, that resulted in a majority favoring the preservation of the union as a renewed federation. Gorbachev's power was greatly diminished after Russian President Boris Yeltsin's high-profile role in facing down a coup d'état by party hardliners. In late 1991, Gorbachev resigned, and the Supreme Soviet of the Soviet Union met and formally dissolved the union. The remaining 12 constituent republics emerged as independent post-Soviet states. Russian Federation—formerly the Russian SFSR—assumed the Soviet Union's rights and obligations and became recognized as the de facto successor state. At the same time, Ukraine by law declared that it is a state-successor of both Ukrainian SSR and the Soviet Union. Today, Russia and Ukraine have an ongoing dispute over formerly-Soviet property.
The Soviet Union produced many significant technological achievements and innovations of the 20th century, including the world's first human-made satellite, the first humans in space and the first probe to land on another planet, Venus. The country had the world's second-largest economy and the largest standing military in the world. The USSR was recognized as one of the five nuclear weapons states and possessed the largest stockpile of weapons of mass destruction. It was a founding permanent member of the United Nations Security Council as well as a member of the Organization for Security and Co-operation in Europe (OSCE), the World Federation of Trade Unions (WFTU) and the leading member of the Council for Mutual Economic Assistance (CMEA) and the Warsaw Pact.